1. Field of the Invention
This invention relates to an image reading apparatus and method for reading an image on a document in a copier, a facsimile, an image scanner, or the like.
2. Description of the Related Art
To read an image from a document on a document table, an image scanner is provided with, for example, an image reading apparatus, which contains a light source that illuminates a document through a reading window of a document table, a line image CCD that performs photoelectric conversion on light reflected from the document, and a carriage that supports the light source and the line image CCD beneath the document table. The line image CCD is made up of a line of photocells. The light source is composed of a straight fluorescent lamp. The straight fluorescent lamp and the line of photocells are placed in parallel to a line extending in a main scanning direction in the widthwise direction of the reading window. The carriage is provided so as to be movable in a sub-scanning direction in the lengthwise direction of the reading window.
In an image reading operation, the image reading apparatus first turns on the light source and then causes the carriage to move forward so as to traverse the reading window. As the movement of the carriage changes the coordinates of a document that reflects the light from the light source toward the line image CCD, the line image CCD senses an image of the document in units of one line by repeating the photoelectric conversion corresponding to the change of the coordinates.
The amount of light emitted from the straight fluorescent lamp tends to be smaller near both its ends than near its center. When the amount of light emitted from the light source is not uniform in the main scanning direction, the photocells of the line image CCD produce output data reflecting the non uniformity of the amount of light emitted. Therefore, the output data of the photocells are normally subjected to shading correction to enable exact image reading. Conventionally, two techniques explained below are generally used for shading correction. The first technique is such that a reference white plate is placed on the back of the document table so as to correspond to the home position of the carriage set outside the reading window, and some of the photocells near one end of the line image CCD are masked. The image reading apparatus obtains the output data of the masked photocells and the output data of the unmasked photocells as reference black data and reference white data, respectively, when the reference white plate is illuminated by the fluorescent lamp. It then determines correction coefficients for the unmasked photocells on the basis of the reference black data and the reference white data. After this, it performs shading correction by multiplying the output data of the unmasked photocells by the correction coefficients, when the document is illuminated as the carriage is moved. The second technique is such that none of the photocells of the line image CCD are masked and the aforementioned reference white plate is provided. The image reading apparatus obtains the output data of the photocells as reference black data when the fluorescent lamp is off, and the output data of the photocells as reference white data when the lamp is on. It then determines correction coefficients for the photocells on the basis of the reference black data and the reference white data. After this, it performs shading correction by multiplying the output data of photocells by the correction coefficients, when the document is illuminated as the carriage is moved.
The first technique, however, has the disadvantage of being unable to guarantee the reliability of the correction coefficients because the correction coefficients for the unmasked photocells are determined on the basis of the reference black data obtained from the masked photocells that are not related to actual image reading. The second technique has the disadvantage of delaying the start of image reading after the document on the reading window is replaced. Although a light source such as a fluorescent lamp requires a long rising time from when the lamp is turned on until the amount of light emitted becomes stable, the light source must be turned off once to determine the correction coefficients for the photocells before image reading. This delays the start of image reading for as long as the rising time after the light source is turned on again. Further, with the second technique, the reliability of the correction coefficients is decreased by external light that hits the light image CCD while the light source is off. In addition, the service life of the light source is shorten by the frequent switching of the light source.